Mass spectrometer



Aug. 14, 1956 w. c. WILEY 2,759,105 MASS SPECTROMETER Filed July 9. 1952 Lu L 2 w q WI Q b R") h- INVENTOR. W/LL/AM C. W/AEY BY A less.

United States Patent MASS SPECTROMETER William C. Wiley, Detroit, Mich, assignor t0 Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application July 9, 1952, Serial No. 297,813

15 Claims. (Cl. 25041.9)

This invention relates to mass spectrometers and more particularly, to mass spectrometers for producing an increased resolutionbetween ions of different mass. The invention also relates to methods of producing such increased resolution.

In certain types of mass spectrometers, ions are produced from the molecules of different gas in an unknown mixture and are collected in a relatively confined region for a period of time. The ions are then withdrawn in pulse form by an accelerating force which directs the ions towards a detector responsive to the ions. The accelerating force operates to impart a greater velocity to ions of relatively light mass than to ions of heavy mass and causes the light ions to reach the detector before the ions of heavy mass, provided that all of the ions have the same charge. By indicating the relative times at which the ions of difierent mass are detected, the masses of the different ions in the unknown mixture and their relative abundance can be determined.

In spectrometers of the above type now in use, it becomes increasingly difiicult to distinguish between ions of adjacent mass units as the masses of the ions increase. For example, it is much more difiicult to distinguish between ions having masses of 200 and 201 atomic mass units than between ions having masses of 20 and 21 atomic mass units. Because of this difiiculty, many spectrometers have only a limited range of mass measurements such as the measurement of mass units of 100 or The time-of-fiight spectrometers that do have a relatively wide range of measurement necessarily have an extremely high cost and a fairly critical operation.

This invention provides a cheap and reliable spectrometer having an operating range materally extended over spectrometers now in use. The spectrometer obtains such extended range by increasing the time required for the ions to travel through a predetermined distance. It provides such an increase in travel time by accelerating the ions to a relatively high speed in a first region of relatively narrow width, then decelerating the ions for travel through a second region of considerable width and finally restoring the speed of the ions to a relatively high value before the ions reach the detector.

An object of this invention is to provide a spectrometer for determining the masses of different ions by measuring the time required for the ions to travel through a predetermined distance.

Another object is to provide a spectrometer of the above character for enhancing the resolution between ions of adjacent mass units and for especially enhancing the resolution between relatively heavy ions of adjacent mass units.

A further object is to provide a spectrometer of the above character for extending the range in which ions of adjacent mass units can be satisfactorily distinguished by delaying the time required for the ions to travel through a predetermined distance.

Still another object is to provide a method of enhancing the resolution between ions of different mass and of ex- Ice tending the range of satisfactory delineation between ions of adjacent mass units.

Other objects and advantages will be apparent from a detailed description of the invention and from the appended drawings and claims.

The single figure is a somewhat schematic view, partly in perspective and partly in block form, illustrating one embodiment of the invention.

In one embodiment of the invention, a wedge-shaped filament 10 made from a suitable material such as tungsten is adapted to emit electrons when heated. An electrode 12 is positioned adjacent the filament 10 and is provided with a vertical slot 14 the median position of which is at substantially the same horizontal level as the filament 10. An electrode 16 having a slot 13 corresponding substantially in shape and position to the slot 14 is disposed substantially in alignment with the electrode 12 at a relatively close distance to the electrode. A collector 20 is positioned at a relatively great distance from the electrode 16 and in substantial alignment with the electrode.

A backing plate 22 is provided between the electrode 16 and the collector 2t and on substantially perpendicular relationship to these members. The plate is positioned slightly to the rear of an imaginary line extending from the tip of the filament 10 through the slots 14 and 18 in parallel relationship to the plate. An electrode '24 having a horizontal slot 26 is disposed is substantial alignment with the plate 22 and slightly in front of the imaginary line disclosed above. For example, the electrode 24 may be separated from the plate 22 by approximately 2 millimeters.

Top and bottom slats 28 extend between the plate 22 and the electrode 24 to form a compartment, and a conduit 3t) communicates with the compartment through a horizontal slot 32 in the bottom slat 28. The conduit 30 extends from a receptacle 34 adapted to hold the molecules of the different gases in an unknown mixture.

An electrode 36 having a slot 38 corresponding substantially in shape and position to the slot 26 is positioned relatively close to the electrode 24 and in substantial alignment with the electrode. For example, the distance between the electrodes 24 and 36 may be approximately 2 millimeters. An electrode 46 preferably formed from a wire mesh is in turn disposed in substantial alignment with the electrode 36 but at a relatively great distance, such as 10 centimeters, from the electrode.

A collector 46 or other suitable detector is in turn positioned at a relatively great distance, such as 10 centimeters, from the electrode 40. A time indicator 50, such as an oscilloscope, is connected to the collector 46 to indicate the relative times at which the ions of different mass reach the collector.

In the steady state condition, a positive voltage is applied from a suitable power supply 52 through a resistance 54.to the electrode 12 and slightly positive voltages are also applied to the collectors 2t) and 46 through suitable resistances 56 and 58, respectively. A slightly positive voltage is applied to the collector 20 so that the collector will attract the electrons flowing past the electrode 16. A slightly positive potential is applied to the collector 46 to produce a force by the collector for attracting back to it electrons secondarily emitted from it by the impingement of ions.

The filament 10, the electrode 16, the backing plate 22 and the electrodes '24, 36 and 40 are at substantially ground potential in the steady state condition. The electrodes 16 and 36 are directly grounded, and the filament 10, the backing plate 22 and the electrodes 24 and 40 may be grounded through suitable resistances 60, 62, 64 and 66, respectively. v

The electrons emitted from the filament 10 are accelerated towards the electrode 12 but are not further accelerated beyond the electrode 12 since the electrode 16 is substantially at the same potential as the filament. This prevents most of the electrons from traveling into the region between the backing plate 22 and the electrode 24 and prevents any electrons which do reach this region from having sufiicient energy to ionize molecules of gas introduced into the region.

When negative pulses of voltage are applied from a pulse forming circuit v68 to the filament and the electrode 12 through suitable coupling capacitances 70 and 72, respectively, the electrode 16 becomes more positive than electrode 12. This causes an additional acceleration to be imparted to the electrons as they travel through the region between the electrodes 12 and 1 6. Because of their additional acceleration, the electrons travel into the region between the plate 22 and the electrode 24 with sufiicient energy to ionize molecules of gas introduced into the region. The ions so produced are retained in the negative potential well created by the electrons flowing towards the collector 20.

Because of the relatively large negative charge of the electron stream, a considerable number of ions can be retained in the stream before the stream becomes saturated. These ions are retained in a space having a relatively narrow width since the electron stream is itself narrow as a result of the collimating action which is provided by the slots 14 and 18 and which may also be provided by a magnetic field (not shown). The action of the electron stream in retaining a considerable number of ions in a relatively confined region is ifully disclosed in co-pending application Serial No. 221,654 filed April 18, 1951, by Ian H. McLaren and William C. Wiley, now Patent No. 2,732,500.

When the number of ions in the electron stream approaches saturation, the electron stream is cut ofi by the removal of the negative pulses of voltage on the filament 10 and the electrode .12. Approximately the same instant or very shortly thereafter, positive pulses of voltage are applied from the circuit 68 to the backing plate 22 and the electrode 24 through suitable coupling capacitances 74 and 7 6, respectively. The positive pulses of voltage cause the ions to be withdrawn in a pulse from their place of storage.

The voltage pulses applied to the plate 22 and the electrode 24 are of such magnitude as to create an electric field of moderate intensity in the region between the plate and the electrode and an electric field of increased intensity between the electrodes 24 and 316. For example, voltage pulses of approximately +200 and +150 volts may be applied to the plate 22 and the electrode 24, and the electrode 36 may be maintained at ground.

Upon the imposition of the electric fields in the region between the plate 22 and the electrode 24 and in the region between the electrodes 24 and 36, ions of the same charge are accelerated by amounts substantially inversely proportionate to their masses. Thus, ions of relatively light mass are given a greater velocity than ions of heavy mass. Because of the differences in velocity, the ions become separated into groups with each group containing ions of a particular mass and with the ions in each group traveling at a speed dependent upon the mass of the ions in the, group.

The moderate electric field in the first region and the increased electric tfield in the second region serve to focus the. ions in each group by providing a. compensation for any difierences in positioning the random motion which the ions may have had while they were retained in the electron stream. The differences in, positioning of: individual ions occur because. of the finite. width. of the electron stream. The differences in the. random motion of individual ions occur because thev thermal. and other energy in the ions causes some of, the ionstobe traveling towards the plate 22 andother ionstobetraveling towards the electrode 24 at the time that'the electron 4 steam is removed. The focusing action provided by the electric fields in the regions between the plate 22 and the electrode 24 and between the electrodes '24 and 36 is fully disclosed in co-pending application, Serial No. 249,318 filed October 2, 1951, by William C. Wiley, no'w Patent No. 2,685,035.

A positive pulse of voltage is applied to the electrode 40 at substantially the same instant as the application of the voltage pulses on the backing plate '22 and the electrode 24. The voltage pulse is applied to the electrode 40 through a suitable coupling capacitance 78 and is of approximately the same order of magnitude as, but somewhat longer duration than, the pulse on the electrode 24. Because of the voltage pulse on the electrode '40, an electric field is created between the electrode 36 and the electrode 40 to decrease the speed of the ions as they travel through the region between the electrodes. The decelerating force produced by the electric field is greater for ions of relatively light mass than for ions of heavy mass. When the ions have moved past the electrode 40, they are repelled by the electrode towards the collector 46 so as to be accelerated towards the collector.

As previously disclosed, the voltage difference between the electrodes 36 and 40 is substantially equal to the voltage difierence between the electrode 40 and the collector 46. Because of this relationship, the ions regain the energy in the region between the electrode 40 and the collector 46 that they lost in the region between the electrodes 36 and 40. This causes the ions to have substantially the same velocity as they impinge on the collector 46 as they had when they traveled past the electrode 36.

The deceleration provided on the ions during their movement between the electrodes 36 and 40 produces an increase in the time required for the ions to reach the collector 46 even though the ions regain their speed as they move past the electrode 40 towards the collector. This increase in the travel time of the ions produces a corresponding increase in the time between the collection of ions of adjacent mass units. Since the time between the collection of ions or adjacent mass units helps to determine the resolution by which ions of different mass units are distinguished, the resolution of the mass spectrometer is enhanced. This enhancement in the resolution of the ions is obtained without any increase in the distance of ion travel. The enhanced resolution is especially important for ions of relatively large mass units.

The increased resolution provided by the spectrometer disclosed above over previous spectrometers can be seen by the following analysis.

Consider an ion of a particular mass M and another ion of a heavier mass M1. The time for M to traverse the first and second fields between the electrode 36- and the collector 46 can be given by the relationship,

X1=the distance of the first region between the electrodes 36 and '40.

X2=the distance of the second region between the electrode 40 and the collector 46.

Eo=tbe energy of the ion at the electrode 36. E1 =the energy acquired by the ion in its travel through the first region between the electrodes 36- and 40. Ez=the energy acquired by the ion in its travel through the second region between the electrode 40andthe collector 42.

The denominators in Equation 1 represent theaverage velocity of the ion M in the first and second regions, respectively. Equation 1 may be simplified "as follows:

1 z 2M M [EO +(EO+E1 X 2 1) 0+ l+- 2) Since the energy of the ion M as it leaves the second region is to be the same as when it entered the first region, we have the relationship,

=the distance between the electrode 36 and the collector 46.

Similarly, the time for the mass M1 to traverse the first and second regions becomes M1 E0 E0+Eo The separation in time between the collection of the ions M and M1 becomes At=ZM -IM Substituting Equations 6 and 7 in Equation 8 X At: 2 M M 9 1 H EU D+E1) K At= 10 EO EO+EM where Equation 10 indicates that the separation in time be tween the collection of the ions increases as the denominator decreases. When no field is imposed between the electrode 36 and the collector 46, as is the case in mass spectrometers now in use, E would be zero and the denominator in Equation 10 would be 2Eo When the voltage applied to the electrode 40 is positive with respect to the electrode 36 to provide a decelerating field as is the case in this invention, E1 would have a negative value and the denominator would be Since the value of the latter denominator is less than the former denominator, it becomes obvious that the time separation between ions M and M1 is greater when a decelerating and an accelerating force are applied in the region between the electrode 36 and the collector 46. The resolution between the ions M and M1 is therefore enhanced.

It should be appreciated that different arrangements than the filament 10, the electrodes 12 and 16 and the collector 20 may be employed to produce a plurality of ions and to store these ions until the time for their withdrawal. Other arrangements than the backing plate 22 and the electrodes 24 and 36 may also be employed to produce an acceleration of the ions towards the collector 46.

Furthermore, various circuits may be suitably used as the pulse forming circuit 68. For example, Model 902 6 of the Double-Pulse Generator manufactured by the Berkeley Scientific Company of Richmond, California, may be used to produce first and second pulses sepa rated from each other by a variable period of time. The first pulse may be applied to the filament 10 and the electrode 12, and the second pulse may have its amplitude modified for application to the backing plate 22 and the electrodes 24 and 36. The pulse forming circuit disclosed in co-pending application Serial No. 288,014 filed May 16, 1952, by Macon H. Miller and William C. Wiley may also be conveniently adapted for use.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

1. A mass spectrometer, including, means for producing a plurality of ions and for storing them in a relatively confined region, means for withdrawing the ions in a pulse and for accelerating the ions through a field of relatively narrow width, a detector, means for decelerating the ions in a first region between the place of ion Withdrawal and the detector, means for accelerating the ions in a second region between the first region and the detector, and means for indicating the relative times at which the ions of different mass are detected.

2. A mass spectrometer, including, means for producing a plurality of ions and for storing them in a relatively confined region, an ion source for withdrawing the ions in a pulse and for accelerating them through a relatively narrow region, a detector located at a relatively great distance from the ion source, an electrode disposed between the ion source and the detector, means for decelerating the ions in the region between the ion source and the electrode to increase the travel time of the ions to the detector, means for accelerating the ions in the region between the electrode and the detector, and means for determining the relative times at which the ions of difierent mass are detected.

3. A mass spectrometer, including, means for producing a plurality of ions and storing them in a relatively narrow region, an ion source for withdrawing the ions in a pulse and for increasing their speed to a relatively high value, a detector located at a relatively great distance from the ion source, an electrode disposed between the ion source and the detector, means for decelerating the ions in the region between the ion source and the electrode -to increase the time required for the ions to reach the detector, means for accelerating the ions in the region between the electrode and the detector to substantially the same speed as their speed upon leaving the source, and means for indicating the relative times at which ions of difierent mass are detected.

4. A mass spectrometer, including, means for producing a plurality of ions and storing them in a relatively confined region, means for producing an electric field in a first region of relatively narrow width to withdraw the ions in a pulse and to produce an acceleration of the ions, a detector, means for producing an electric field in a second region between the first region and the detector to slow down the ions traveling towards the detector, means for producing an electric field in a third region between the second region and the detector to increase the speed of the ions traveling towards the detector, and means for indicating the relative times at which the ions of difierent mass are detected.

5. A mass spectrometer, including, a backing plate, a first electrode disposed at a relatively close distance to the backing plate, means for producing a plurality ot ions and for storing them in a relatively confined region between the backing plate and the first electrode, a second electrode disposed at a relatively great distance from the first electrode, a detector disposed at a relatively great distance from the secondelectrode, means for applying a pulse of voltage on the backing plate relative to the voltage onithe first electrode to produce an acceleration ofthe ions between the backing. plate and the first electrode, means for applying a pulse of voltage on the secndv electrode relative to the voltages on the first electrode and the detector to decelerate the ions in the region between the first and second electrodes and to accelerate the ions between the second electrode and the detector, and means for indicating the relative times at which the ions of difierent mass reach the detector.

6. A mass spectrometer, including, a backing plate, a first electrode disposed at a relatively close distance to the backing plate, a second electrode disposed at a relatively great distance from the backing plate, a detector disposed at a relatively great distance from the second electrode, means for applying a pulse of voltage between the backing plate and the first electrode to accelerate the ions towards the first electrode, means for applying a' pulse of voltage on the second electrode relative to the voltage on the first electrode and the detector to pro,- duce a deceleration of the ions in the region between the first and second electrodes and an acceleration of the ions between the second electrode and the detector substantially equal to their deceleration between the first and second electrodes, and means for providing an indication of the relative times at which the ions of different mass are detected.

7. A mass spectrometer, including, a backing plate, a first electrode, means for providing a plurality of ions between the backing plate and the first electrode, an electrical circuit for applying a relative voltage between the backing plate and the electrode to produce a movement of the ions from their place of provision and a separation of the ions on the basis of their mass, a detector disposed at a particular distance from the first electrode, a second electrode disposed between the first electrode and the detector at particular distances from these members, an electrical circuit for applying a relative voltage between the first and second electrodes to slow the ions down during their movement towards the second electrode and for applying a relative voltage between the second electrode and the detector to speed up the ions during their movement towards the detector, and means for indicating the'relative times at which the ions of difierent mass are detected. e

8. A mass spectrometer, including, means for providing a plurality of ions, means for providing a first region of finite length, means for providing a second region of particular length in proximity to the first region, means for providing a third region of particular length in proximity to the second region, a detector disposed at the end of the third region, means for producing an increase in velocity of the ions through the first region to produce a separation of the ions on the basis of their mass, means for producing a decrease in velocity of the ions through the second region to prolong the time of ion travel towards the detector, means for producing an increase in velocity of the ions through the third region to at least a substantial portion of their velocity at the end of the first region, and means for indicating the relative times at which the ions of diflerent mass are detected.

9. A mass spectrometer, including, means for providing a plurality of ions, means for imparting energy to the ions in a first region of finite length to produce a movement of the ions from their place of ion provision and to produce a separation of the ions on the basis of their mass, means for reducing the energy of the ions in a second region of particular length, means for restoring to the ions in a third region of particular length at least a substantial part of the energy that they had at the end of the first region, means for detecting the ions after their travel through the thirdregion, and means for indicating the relative times at which the ions of difierent mass are detected.

10. A mass spectrometer, including, means for providing a plurality of ions, means for providing a first region of finite length, means for providing a second region of particular length after the first region, means for providing a third region of particular length after the second region, a detector disposed at a particular distance past the third region, an electrical circuit for applying an electrical field on the ions in the first region to produce a separation of the ions on the basis of their mass and to impose a velocity on the ions, an electricalcircuit for imposing an electrical field on the ions in the second region to produce a decrease in the velocity of the ions in the region for an increase in the time required for the ions to produce signals at the detector, an electrical circuit for imposing an electrical field on the ions in the third region to restore the velocity of the ions to at least a substantial fraction of their velocity at the end of the first region, and means for indicating the relative times at which the ions of difierent mass are detected.

11. A mass spectrometer, including, a backing plate, a first electrode, means for providing a plurality of ions between the backing plate and the first electrode, an electrical circuit for producing an electrical field between the backing plate and the first electrode to produce a withdrawal of the ions from their place of provision and a separation of the ions on the basis of their mass, a detector located at a particular distance from the'first electrode, a second electrode disposed at an intermediate distance between the first and second electrodes, an electrical circuit for producing an electrical field in a region between the first and second electrodes to decelerate the ions and an electrical field in a region between the secondv electrode and the detector to accelerate the ions, and means for indicating the relative times at which the ions of different mass are detected.

12. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate, a detector disposed at a particular distance from the first electrode, a second electrode disposed at an intermediate position between the first electrode and the detector, means for providing a plurality of ions between the backing plate and the first electrode, means for imposing a force on the ions in the region between the backing plate and the first electrode to'produce a movement of the ions towards the electrode and to produce a separation of the ions on the basis of their mass, means forimposing a force on the ions in a region between the firstand second electrodes to produce a deceleration of' the ions for a prolongation of the time required for the ions to reach the detector, means for imposing a 'force on the ions in a region between the second electrode and the detector to restore the ionsto at least a velocity approaching their velocity in the region between the backing plate and the first electrode, and means for indicating'the relative times at which the ions of difierent mass are detected.

13. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance fromthe backing plate, a second electrode disposed at a particular distance from the first electrode, means for providing a plurality of ions; in the region between the backing plate and the first electrode, means for imposing relative voltages on the backing plate and the first electrode to provide the ions with moderate velocities during their travel throughthe first region and to provide the ions positioned closer tothe backing plate with slightly greater velocities than the ions closer to the-electrode, means for imposing relative voltages onthe first and second electrodes to provide the ions of each mass with substantially constant increments in energy and with increments in energy greater than those previously imparted to the ions, a detector disposed at a particular distance past the second electrode, a third electrode disposed. between the second electrode and the detector, means for imposing relative voltages on the second and third electrodes to slow down the ions in the region between the second and third electrodes for the prolongation of the time required for the ions to reach the detector, means for imposing relative voltages on the third electrode and the detector to provide the ions with at least a substantial fraction of their energy during their movement towards the second electrode, and means for indicating the relative times at which the ions of different mass are detected.

14. A mass spectrometer, including, means for providing a plurality of ions, means for providing a first region of finite length, means for providing a second region of particular length after the first region, means for providing a third region of particular length after the second region, means for imposing a force on the ions in the first region to accelerate the ions from their place of provision and to produce a separation of the ions on the basis of their mass, means for imposing a force on the ions in the second region to decelerate the ions, means for imposing a force on the ions in the third region to accelerate the ions to a velocity approaching the velocity of the ions at the end of the first region, a detector disposed at a particular position past the third region, and means for indicating the relative times at which the ions of different mass are detected.

15. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate, a second electrode disposed at a particular distance from the first electrode, means for providing a plurality of ions in the region between the backing plate and the first electrode, an electrical circuit for applying an electrical field between the backing plate and first electrode of a first polarity and of moderate intensity until the movement of the ions past the first electrode, an elecrical circuit for applying an electrical field of the first polarity and of considerably increased intensity in a region between the first and second electrodes until the movement of the ions past the region, a detector disposed at a particular distance past the second electrode, a third electrode disposed between the second electrode and the detector, an electrical circuit for applying an electrical field in a region between the second and third electrodes of opposite polarity until the movement of the ions past the region, an electrical circuit for applying in a region between the third electrode and the detector an electrical field of the first polarity and of a magnitude to restore the velocity of the ions to at least a substantial fraction of their velocity during their movement towards the second electrode, and means for indicating the relative times at which the ions of difierent mass are detected.

References Cited in the file of this patent UNITED STATES PATENTS 2,582,216 Koppius Jan. 15, 1952 2,612,607 Stephens Sept. 30, 1952 2,642,535 Schroeder June 16, 1953 

1. A MASS SPECTROMETER, INCLUDING, MEANS FOR PRODUCING A PLURALITY OF IONS AND FOR STORING THEM IN A RELATIVELY CONFINED REGION, MEANS FOR WITHDRAWING THE IONS IN A PULSE AND FOR ACCELERATING THE IONS, THROUGH A FIELD OF RELATIVELY NARROW WIDTH, A DETECTOR, MEANS FOR DECELERATING THE IONS IN A FIRST REGION BETWEEN THE PLACE OF ION WITHDRAWAL AND THE DETECTOR, MEANS FOR ACCELERATING THE IONS IN A SECOND REGION BETWEEN THE FIRST REGION AND THE DETECTOR, AND MEANS FOR INDICATING THE RELATIVE TIMES AT WHICH THE IONS OF DIFFERENT MASS ARE DETECTED. 